Aminotetralins are compounds having remarkable sympathomimetic activity.
Thanks to their properties, they can be used in different therapeutical fields as medicaments active at the bronchial, cardiovascular, renal and central nervous system levels.
A number of methods for the preparation of 5,6-ADTN are described in literature or in patents starting from the end of the 60s.
Some syntheses require the preparation of 5,6-dimethoxy-1-tetralone which is then transformed into 5,6-ADTN by means of various procedures: by Neber rearrangement of 5,6-dimethoxy-1-tetralone O-tosyl oxime (W. K. Sprenger et al., J. Med. Chem., 12, 487, 1969 modified by J. C. Kim et al. J. Nor. Chem. Soc., 21(3), 187, 1977), by .alpha.-bromination and subsequent substitution of bromine (J. C. Kim et al., J. Kor. Chem. Soc., 20, 91, 1976; Y. Oka et al., Chem. Pharm. Bull., 25(4), 632, 1977), by .alpha.-nitrosation and subsequent reduction (Y. Oka et al., Chem. Pharm. Bull., 25(4), 632, 1977).
Some syntheses require the preparation of 5,6-dimethoxy-2-tetralone which is then transformed into the corresponding 2-amino derivative by reductive amination (J. D. McDermed et al., J. Med. Chem., 18(4), 362, 1975; J. G. Cannon et al., J. Med. Chem., 20(9), 1111, 1977; U.S. Pat. No. 646,300, 1976 (Iowa Univ.) or by transformation into the corresponding O-methyloxime and subsequent reduction (J. G. Cannon et al., J. Med. Chem., 17(5), 565, 1974).
Some syntheses are based on the preparation of 5-hydroxy-6-methoxy-1,2,3,4-tetrahydro-2-naphthoic acid and the subsequent transformation into 5,6-ADTN by Curtius rearrangement (K. Mitsuhashi et al., Chem. Pharm. Bull., 20(6), 1321, 1972).
The only enantiospecific synthesis of 5,6-ADTN is based on the use of 2,2-dimethyl-3-methoxycarbonyloxazolidine-4-aldehyde (A. D. Baxter et al., Tetrahedron Letters, 33(17), 2331, 1992).
All these methods suffer from severe restrictions for the industrial use.
Recently, a method for the preparation of 2-amino-5,6-dimethoxytetralin applicable industrially has been claimed in EP-A-0 534 536, filed on Sep. 17, 1991, in Zambon's name.
According to the latter reference, 5,6-dimethoxy-2-aminotetralin is obtained through the key intermediate, i.e. the corresponding 1-tetralone, already widely described in the literature cited above. The tetralone is obtained starting from condensation of 2,3-dimethoxybenzaldehyde with pyruvic acid (step A), transformation of the keto group into amino group and reduction of the unsaturated amino acid (step 3) and intramolecular cyclization to give the desired 5,6-dimethoxy-2-amino(protected)-1-tetralone.
The condensation reaction (A) had already been described by Hudson et al., J. Chem. Soc., 715-722 (1941) and Pavel et al., Acta Univ. Palacki Olomuc. Fac. Rerum. Natur., 401-404 (1971).
On the subject, EP 0 534 536 stresses that such a reaction was carried out in the presence of an inorganic base, such as sodium hydroxide (Hudson) or potassium carbonate (Pavel) with very low yields (respectively 40 and 6%), and therefore it showed no industrial interest.
In EP 0 534 536 cited above, the problem of the low yield is overcome by carrying out the condensation in the presence of an organic base, such as triethylamine, piperidine, piperazine and morpholine. The condensation product is obtained in yields up to about 80%. The condensation described therein requires particular reaction conditions, such as an anhydrous organic solvent (dimethylformamide is cited), inert atmosphere, low temperatures during the reagent addition phase. The recovery of the product requires an extraction procedure to remove the organic solvent.
From the industrial point of view, the condensation described above has additional costs in terms of reagents, anhydrous solvents and plant requirements.
Now it has been found that the condensation reaction of 2,3-dialkoxybenzaldehyde and pyruvic acid allows to obtain 4-(2,3-dialkoxyphenyl)-2-ketobutenoic acid in high yields, when carried out in the presence of an inorganic base in an aqueous/alcoholic solvent. The reaction is carried out at room temperature.